Approximate how thick is the skin?
1-2 mm
The integumentary system includes:
- Skin (cutaneous membrane)
- epidermis
- dermis
- Accessory structures (hair, oil & sweat glands, sensory receptors).
Functions of integumentary system.
- Regulates body temperature
- Stores blood
- Protection from external environment
- Excretion and absorption
- Sensation
- Vitamin D synthesis
Epidermis vs Dermis
Epidermis
- more superficial, thinner
- contains epithelial tissue
- somewhat avascular
Dermis
- deeper and thicker
- contains connective tissue
- vascularized
Subcutaneous layer
AKA hypodermis
Not part of skin
Consists of areolar and adipose tissue
Functions: fat storage, blood vessel passage and location of pressure sensing nerve endings.
The epidermis is primarily composed of what tissue type?
Stratified squamous epithelium
4 principle cells of the epidermis
1 keratinocytes
2 melanocytes
3 Langerhans cell
4 Merkel cells
Keratinocytes
90% of epidermal cells
Produce keratin and lamellae granules
Keratin
Protein produced by keratinocytes
Protects skin and underlying tissue from heat, microbes and chemicals
Originate as tonofilaments which are converted by keratohyalin into keratin in the stratum granulosum.
Lamellae granules.
Released by keratinocytes in stratum granulosum and stratum corneum
Produce water repellant sealant
Decreases entry of water and foreign materials (and water loss too).
Melanocytes
In epidermis
Produce melanin
Melanin
Pigment produced by melanocytes and transferred to keratinocytes
Absorb damaging UV light
Shield nucleus of keratinocytes
Langerhans Cells
AKA Intraepidermal Macrophage cells
Epidermal dendritic cells
Arise from RBM and migrate to epidermis.
Participate in immune responses against invading microbes. Antigen presenting.
Merkel Cells
AKA tactile epithelial cells
Least numerous epidermal cell
In deepest layer of epidermis.
Contact Merkel/tactile disc. Detect touch.
Five layers of the epidermis
- Stratum basale/germinativum
- Stratum spinosum
- Stratum granulosum
- Stratum lucidum (palms and soles)
- Stratum corneum
Stratum basale
AKA stratum germinativum
Deepest epidermal layer
SINGLE row of cuboidal or columnar keratinocytes
Also contain melanocytes, Merkel cells and stem cells (which produce keratinocytes)
If this layer is destroyed skin cannot regenerate
Cells attach to each other by desmosomes and to basement membrane by hemidesmosomes
Stratum spinosum
8-10 cells layers held together by desmosomes
Second deepest layer or epidermis
Mostly keratinocytes but also Langerhans cells and melanocyte projections.
Stratum granulosum.
Epidermis
3-5 layers of flat cells undergoing apoptosis
Transition layer between deeper active strata and superficial dead cells.
Keratinocytes release lamellae granules.
Contain keratohyalin which converts tonofilaments into keratin
Stratum lucidum
Layer of epidermis only found in finger tips, palms and soles of feet.
4-6 layers of flat dead cells.
Add additional toughness
Stratum corneum
25-30 layers of flat dead cells filled by keratin and surrounded by lipids
Most superficial layer of epidermis
Contain lamellar granules.
Protect against light, heat, chemicals and bacteria
Callus
Abnormal thickening of the stratum corneum of the epidermis due to friction.
Keratinization
Replacement of cell contents with keratin as cells move to skin surface
Takes 4-6 weeks.
Stem cells product keratinocytes.
Keratinocytes contain tonofilaments which are converted to keratin by keratohyalin in the stratum granulosum.
Epidermal growth factor (EGF) involved.
Dermis is composed primarily of what tissue type?
Dense irregular connective tissue containing collagen and elastin fibres
The Dermis contains:
Fibres (collagen and elastin)
Cells (fibroblasts, macrophages, fat cells)
Hair follicles, glands, nerves and blood vessels.
Elasticity
Ability to return back to the original shape
Extensibility
Ability to stretch
Tensile strength
Ability to resist pulling or stretching.
Two major regions of the dermis
Papillary and reticular
Papillary region
Top 20% of dermis
Thin collagen and elastic fibres
Mostly fibroblasts
Has dermal papillae (epidermal ridges)
Dermal papillae
AKA dermal ridges
Anchor papillary region of dermis to epidermis.
Increase surface area of papillary region
Contain capillaries that feed epidermis
Contain Meissner’s corpuscles (touch) and free nerve endings that sense heat/cold/pain/tickling/itch
Reticular region
Deeper 80% of dermis
Attached to subcutaneous layer.
Consists of:
- thick collagen fibres
- fibroblasts
- wandering cells
- some adipose cells
- blood vessels, nerves, sebaceous and sudiferous glands, ducts and hair follicles.
Epidermal ridges
In the dermis. Not the epidermis. Doh!
Make fingerprints.
Also increases friction, and increases number of Meisnner corpuscles and hence tactile sensitivity.
Skin colour is due to what pigments?
(All in dermis)
Melanin
Carotene
Hemoglobin
Freckles vs moles
Freckles: excess melanin
Moles: excess melanocytes
Albinism
Inability to produce tyrosinase and thus melanin.
Vitiligo
Loss of melanocytes
Jaundice
Buildup of bilirubin
Cyanosis
Bluish tinge due to low oxygen
Pallor
Paleness due to low blood level (shock, anemia)
Development of accessory structures of skin
Develop from embryonic epidermis.
Cells sink inward to form hair, nails and glands.
Hair consists of:
- Shaft (projects above skin)
2. Root (below skin surface)
What are the three layers of hair (shaft and root)?
Medulla
Cortex
Cuticle
Epithelial root sheath
AKA the hair follicle
External root sheath + Internal root sheath
Exists on root only.
Downward continuation of epidermis onto hair root.
External root sheath
Hair bulb
Base of each hair follicle
Nourishes growing follicle
Contains hair matrix
Hair matrix
Germinal layer of cells contained in hair bulb.
Site of cell division.
Arises from stratum basale.
Arrector pili
Smooth muscle in dermis
Contracts with cold or fear
Creates goose bumps
Hair root plexus
Dendrites of neurons that detect hair movement and sense touch.
Normal rate of hair loss
70-100 hairs/day
Three stages of hair growth
Growth stage
Regression stage
Resting stage
Growth stage of hair growth
2-6 years
Matrix cells active producing length
At any time 85% of hair in growth stage.
Regression stage of hair growth
2-3 weeks
Matrix cells stop dividing, follicles atrophy and hair stops growing.
Resting stage of hair growth
3 months.
Afterwards, new growth stage begins
Lanugo
Fine, nonpigmented, downy hair covering fetus.
Terminal hair
Replaces lanugo prior to birth
Course, pigmented hair on eyebrows, eyelashes, scalp.
Velus hair
Short, fine, pale body hair. Replaces lanugo over body.
What happens to hair at puberty?
In response to androgens, terminal hairs replace vellus hairs at axilla, private bits, etc.
In adults: males 95% terminal; females 35% terminal
Hair colour: causes
Dark hair: true melanin
Blonde and red: melanin with iron and sulfur
Graying hair: decline in melanin production
White hair: lack of melanin and air bubbles in medullary shaft.
Two types of melanin
Eumelanin: brown to black
Pheomelanin: yellow to red
Alopecia
Partial or complete loss of hair
Hirsutism
Excessive body hair linked to excess androgen.
Three components of a nail
- Nail body
- Free edge
- Nail root.
Decubitus ulcer
Pressure sore
Deficiency of blood flow causes tissue ulceration.
Erythema
Redness of skin due to enlargement of capillaries in dermis.
Inflammation, infection, allergy, burn.
Psoriasis
Chronic skin condition characterized by rapid cell division and movement of keratinocytes through epidermal area.
Also production off abnormal keratin.
Striae
Stretch marks
Dermal blood vessels rupture when skin is overstretched and bonds between collagen fibres break. Results in silvery white streaks.
Dandruff
Excessive amount of keratinized cells shed from scalp.
Tension lines
AKA lines of cleavage.
Orientation of collagen fibres in reticular region. Indicate predominant direction of underlying collagen fibres.
Age spots
AKA liver spots
Accumulation of melanin over time due to sun damage.
Nail body
Visible portion of nail
Lunula
Whitish “half moon” at proximal end of nail.
Capillaries not visible through thickened epithelium.
Free edge
Part of nail that extends past digit
Nail root
Part of nail buried in fold of skin
Function of nails
- Protection
- Counter pressure
- Grasp and manipulate small objects
- Scratching and grooming.
Hyponychium
Nail bed
Thickened stratum corneum.
Attaches nail to fingertips
Eponychium
Cuticle
Narrow band of epidermis (stratum corneum) that extends from and adheres to lateral side of nail wall.
Nail matrix
Proximal portion of epithelium deep to nail root whose cells divide mitotically
Transformation of superficial nail matrix cells into new nail cells –> how nails grow. (Average 1 mm/week)
Glands of the skin
Exocrine glands in the dermis
Four types of skin glands
1 sebaceous
2. Sudiferous
3 ceruminous
4. Mammary
Sebaceous glands
Oil glands. Produce sebum.
Less active before puberty.
Opens into neck of hair follicle.
Absent in palms and soles.
Sebum
Released by sebaceous glands.
Cholesterol, proteins, fats and salts.
Moistens, waterproofs, protects agains bacteria and fungi.
Sudiferous glands: two types
Sweat glands.
Eccrine
Apocrine
Eccrine glands
Ducts terminate at sweat pores on skin surface.
Develop and function soon after birth.
Functions:
- Thermoregulation
- Emotional sweating
- Waste elimination (urea, ammonia, salt)
Apocrine glands
Sudiferous glands that start working at puberty.
Axilla, groin, areolae, etc
Sweat is cloudy and smelly.
Ceruminous glands
Makers of ear wax (cerumen) = combination of ceruminous and sebaceous secretions.
Thermoregulation
Homeostatic regulation of body temperature
Stages of epidermal wound healing
Abrasion, minor burn
Basal cells migrate across wound
Contact inhibition
Epidermal growth factor stimulates basal cell division
Phases of deep wound healing
- Inflammatory
- Migratory
- Proliferative
- Maturation.
Inflammatory phase
Clot unites wound edges and WBCs arrive from dilated and more permeable blood vessels.
Migratory phase
Regrowth of epithelial cells begins and start of scar tissue formation by fibroblasts.
Wound fills with Granulation tissue.
Proliferative phase
Completion of tissue formation.
Random laying down of collagen fibres.
Continued revascularization
Maturation phase of healing
Scab falls off
Collagen becomes more organized.
Fibroblasts disappear.
Fibrosis
Scar tissue formation
Collagen fibres more densely arranged.
Decreased elasticity
Fewer blood vessels
Epidermis derives from which germ layer.
Ectodermal
Leads to hair, nails, skin.
Dermis arises from which germ layer.
Mesenchymal.
Blood tissue and connective tissue too.
Vernix caseosa
“Cheese varnish”
Slippery oil coating on neonates. Protects from amniotic fluid.
UVA rays
Penetrate furthest into skin.
Absorbed by melanocytes.
Depress immune system.
UVB rays
Don’t penetrate as deeply as UVA rays.
Cause sunburns and most tissue damages.
Basal cell carcinoma
78% of all skin cancers
Tumours in stratum basale
Rarely metastasize.
Squamous cell carcinoma
20% of all skin cancers.
Tumours in stratum spinosum
May metastasize.
Malignant melanoma
Affects melanocytes.
Major burn
3rd degree burn over 10% of body, or on face, hands, feet or perineum
2nd degree burn over 25% of body.
First degree burn
Only epidermis
Second degree burn
Destroys entire epidermis and part of dermis
Characterized by fluid filled blisters
Heals without grafting in 3-4 weeks
Third degree
Destroys epidermis, dermis and epidermal derivatives and subQ layer
Most skin functions, sensation lost.